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Strmé InAs/GaAs heteropřechody a velmi tenké kvantové jámy vypěstované pomocí MOVPE
Hulicius, Eduard ; Pacherová, Oliva ; Oswald, Jiří ; Hospodková, Alice ; Pangrác, Jiří ; Šimeček, Tomislav ; Melichar, Karel ; Petříček, Otto ; Chráska, T. ; Holý, V. ; Vávra, I. ; Ouattara, L.
We have found that the abruptness and symmetry on monoatomic scale is achievable with MOVPE technique as well. We are able to find only the periodicity in these structures while atomic resolution pictures give reliable thickness values.
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Nanocompositní materiál s železnými nanočásticemi obalenými grafitem připravený dvoustupňovou procedurou
David, Bohumil ; Pizúrová, Naděžda ; Schneeweiss, Oldřich ; Bezdička, Petr ; Alexandrescu, R. ; Morjan, I. ; Cruneteanu, A. ; Voicu, I.
We describe the phase composition and magnetic properties of the nanocomposite based on iron/graphite core-shell structured nanoparticles. This nanocomposite was prepared by a two-step procedure: in the first step, the original Fe-C-N based nanopowder was synthesized by the method of laser pyrolysis of gas phase reactants, and subsequently in the second step, it was heated up to 800°C in vacuum. Annealing in vacuum induced the formation and growth of iron/graphite core-shell structured nanoparticles. Both the originally synthesized Fe-C-N based nanopowder and the annealed nanopowder were characterized by TEM, XRD and magnetic measurements. The presence of iron nanoparticles with the mean diameter of 40 nm in the annealed state of the nanopowder was proved by XRD and TEM analysis. Mössbauer spectroscopy was used to observe a qualitative change in phase composition.
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Preparation and properties of nanocomposites based on Zr-Fe
Roupcová, Pavla ; Schneeweiss, Oldřich ; Bezdička, Petr
The spark erosion of the pure Zr and Fe electrodes in hydrogen yielded nanocrystalline powder consisting of the mixture of Zr-rich (Zr3FeH7, ZrO2, and Zr(Fe)) and Fe-rich phases with the mean chemical composition Zr-3.2wt.%Fe. Heat treatment in hydrogen atmosphere at 973K induced phase decomposition and transitions with the final composition of bcc Fe, Zr3Fe, ZrH2 and dominating ZrO2 phases. The result indicates that formation of Fe-rich phase (bcc Fe) embedded in ZrO2 could be possible using an appropriate heat treatment of the as-prepared powder.
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Preparation and properties of MgO-Ni(Fe) nanocrystalline composite
Schneeweiss, Oldřich ; Pizúrová, Naděžda ; Jirásková, Yvonna ; Žák, Tomáš ; Bezdička, Petr ; Večerníková, Eva
We have prepared MgO-Ni(Fe) nanocrystalline composites by controlled heat treatment of compacted spark synthesized nanocrystalline powders. Besides particle sizes and their chemical compositions, influence of interaction with surrounding atmosphere and adsorbed gases on phase composition and magnetic properties were investigated. Mössbauer spectroscopy, x-ray diffraction and TEM were used for phase analysis. Thermomagnetic curves and DTA/TG measurements were applied for detection of changes in structure and phase composition during heat treatments. The simultaneous gas analysis using mass spectroscopy by heating yielded information on adsorbed and dissolved gases and their reactions with the nanoparticles.
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Influence of annealing conditions on Mossbauer spectroscopy parameters of Fe-Zn intermetallic phases
Zmrzlý, Martin ; Schneeweiss, Oldřich ; Houbaert, Y. ; Fiala, J.
The phase composition of galvannealed zinc coatings strongly depends on used annealing conditions. According to equilibrium binary phase diagram of iron and zinc, it is possible to find four intermetallic phases. They arise due to diffusion of zinc and have some certain limits of stability. After dipping into molten zinc the steel sheet proceeds to annealing furnace. Since this is the diffusion controlled and thermodynamically determined process, the composition of coating differs with temperature and time of annealing. Also within the phases themselves there are small but important differences of space arrangement of unit cells, what is clearly observable at Mossbauer spectra parameters ů isomer shifts, quadrupole splittings and relative areas of peaks.
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